The present invention relates to a femoral stem for hip prosthesis used for replacing the diseased natural joint of the hip with an artificial device taking its functions.
It is well known that in the orthopaedic field surgery of the hip joint is generally carried out for old people to treat pathologies such as arthrosis, arthritis or hip luxation generating a progressive wear of the hip joint.
It is well known that the hip joint connects the femur to the pelvis and comprises the femur head engaged in the acetabulum which is a joint cavity on the outer face of the pelvic bone.
In the femur in addition to said femoral head there are the neck connecting the head to the femoral body, having a rather longitudinal development, and a couple of protrusions, the greater and the lesser trochanter, arranged on opposite sides on the femoral body.
On the basis of the natural shape of the hip, the prosthesis to be implanted comprises therefore a femoral part called stem and made of metal, generally of a titanium alloy or stainless steel, inserted into a longitudinal cavity made in the femoral body and an acetabular part called cotyle which is applied into a seat made in the pelvis.
The cotyle is generally consisting of two parts, the first made of metal, titanium or its alloys, and the other facing the stem made of polyethylene of high molecular weight or ceramics.
The cotyle is coupled with a spherical head also made of metal or ceramics, which is fixed to the stem and is the pivot of the artificial joint.
More particularly the femoral stem comprises a main body known also as shaft which is inserted in the femur body. Such a shaft ends at the trochanter zone with a shoulder while continuous medially with a neck projecting from the shoulder to the cotyle of the prosthesis and having a terminal cone to which the above mentioned spherical head is applied.
The surgical operation to the patient's hip therefore consists in preparing in the acetabular zone of the pelvis using proper spherical mills, a seat in which the cotyle is then applied.
Then the neck is being cut with a saw, said operation being called resection or ostheotomy and in the subsequent preparation through proper rasps, of the internal canal of the femoral bone where the prosthetic stem is then inserted.
Finally the spherical head is assembled with the femoral stem thus assembling the prosthesis and restoring the original configuration of the limb.
In this way once implanted, the femoral stem and the cotyle reproduce almost faithfully the original shape of the femur and the acetabulum respectively, replacing the diseased parts, allowing a pain relief and a recovery of the joint function so as to allow patient to have a normal life for many years.
The prosthesis is anchored to the bone through two main methods, the first consisting in solidification during surgery of a liquid polymer that in this way acts as a glue, connecting firmly the bone to the prosthesis. In this case a cemented prosthesis is obtained.
The prosthesis of most modern conception are however pressure applied into the bone, to which they are stably anchored through a natural process of bone integration occurring with time, enhanced also by the fact that the metal stem is rather sanded, that is has a rough outer surface, thus increasing the anchoring ability of the bone to the stem.
The present invention intends to address this second kind of prosthesis.
A wide variety of embodiments of femoral stems for hip prosthesis are presently available on the market, performing in different ways the object aimed to.
The main limitation of the femoral stems of the prior art consists in that they are rather rigid, above all in relation to the intrinsic elasticity characteristics of the bones of the human body and more particularly of the femur.
The application of a rigid metal body inside the femur generates indeed physical decompensations to the person, above all in their areas of greater load transmission such as the cortical medial areas which become therefore particularly critical areas.
The transmitted stresses are not properly absorbed by the cortical bone which is of the compact kind, with the consequence of causing pain to the person but more particularly causing degeneration or even progressive disappearance of the bone in the less stimulated zones, among which a particular place is to be given to the Calcar located immediately above the lesser trochanter.
Another limitation of the femoral stems of the prior art is their high dimension, more particularly at the shoulder, contributing to making the entire structure stiff.
The solution to make less cumbersome stems reducing the shoulder dimension, is not always possible because at this point the torsional stability of the prosthesis is strongly reduced.
Moreover post operative radiographs show for these constructional types a line of detachment between bone and prosthesis at the lateral shoulder of the stem.
It is clear that this aspect has a negative influence on the performance of the prosthetic implant because the stem results to be partially detached from the femur, not to speak of the clear troubles that this situation causes for the patient's health.
A further attempt to solve the problem of the stiffness of the prosthetic stem inside the femur is disclosed in the document EP 308297, wherein the stem has a notch of different shape made starting from the surface connecting the shoulder with the neck and developed downwards in the proximal zone.
Laboratory test of mechanical resistance show however that after having being subject to a set of load cycles, this femoral stem fails by fatigue at the notch end, where a critical section is generated caused by an excessive concentration of stress. In this connection it is to be recalled that there is a European regulation (UNI7206) according to which the femoral stem of hip prosthesis must withstand five millions load cycles to be approved.
A second drawback of the stem disclosed in said document is that during the load only the part facing the lesser trochanter bends downwards, while the shoulder remains stationary thus preventing to keep the greater trochanter reactive.
Another drawback of this stem is due by the fact that the notch as shaped allows that chips produced by rubbing of the spherical head against the cotyle, to deposit by gravity inside through the upper opening, this leading with time to prosthesis failure.